1. Field of the Invention
The present invention relates to a technique for recovering the contents of cells operating an asynchronous transfer mode (ATM) exchange when being abnormally inputted into the ATM exchange, and more particularly to an apparatus and method for recovering abnormal control cells in an ATM exchange subscriber unit, wherein internal control cells and signal cells to be processed by a subscriber interface are recovered rapidly and effectively when being abnormally transferred to the subscriber interface.
2. Description of the Related Art
An exchange generally functions to form a signal transfer path between two or more users so as to exchange conversation and data information therebetween.
The exchange inputs a subscriber signal and connection request signal at an ingress line, and separates the connection request signal from the subscriber signal to detect the connection request signal. Thereafter, the exchange selects an egress line through which the subscriber signal is to be transmitted, and sets up a path between the ingress line and the egress line.
Upon detecting no signal transfer or a connection release signal while monitoring the ingress line-egress line path set up as described above, the exchange releases the setup path and calculates a fee for the path occupancy at a certain rate to charge the fee.
The above-mentioned exchange may be classified according to types of information or signals being transferred. For example, the exchange is called a telephone exchange when information or signals being transferred are of a voice type such as a conversation, a telegraph exchange when of a character type such as a message, and a data exchange when of an image information type.
On the other hand, an asynchronous transfer mode (ATM) is a transfer mode of the B-ISDN (broadband ISDN) determined in ITU-T (the successor of CCITT), 1988. This transfer mode is a transfer and switching technique that is the kernel of the B-ISDN.
The ATM serves to partition all data information to be transferred between exchanges into ATM cells, which are block or packet units with fixed data lengths, and transfer the partitioned ATM cells in order.
An ATM cell has a fixed length of 53 bytes, the first 5 bytes forming a header and the remaining 48 bytes forming an information field or payload field. This fixed length ATM cell or data stream is a basic unit of multiplex communication or multiplex switching.
Written in the 5-byte header are start of cell (SOC) information indicative of a start point of the ATM cell, a virtual channel identifier (VCI) for identification of connection information associated with the cell, a virtual path identifier (VPI), a cell loss priority (CLP) indicative of cell discard allowance or disallowance when a congestion occurs in signals to be transferred, payload type identification (PTI) information for identification of network control information, header error control (HEC) information for detection and control of a header error, etc.
Statistics show that the ATM-based multiplex switching is characterized in that it can obtain a higher multiplexing efficiency than time division multiplex switching and freely vary/set transfer bandwidths to be assigned to respective communications.
Because the routing information as stated above is written in the cell header, an ATM exchange can relay and switch payload cell contents by itself by analyzing the cell header. Further, the relay or switching operation can be realized in a hardware manner, thereby enhancing a switching rate.
The ATM exchange has line units of two levels, a virtual path (VP) and a virtual channel (VC) The ATM exchange also performs a packet switching operation of a high transfer efficiency to settle problems with a circuit switching operation, such as a switching delay and a degradation in circuit utilization efficiency, thereby processing a variety of information at high speed.
The ATM exchange processes cells that are roughly classified into three types, a user cell, an internal control cell and a signal cell.
The user cell is composed of a voice or data signal transferred from a subscriber to the ATM exchange, which signal has payload cell contents.
The internal control cell may be defined and operated in different manners with respect to respective ATM exchanges. Generally, the internal control cell is used for internal communication between a control unit that manages the entire operation of the ATM exchange and each function block or function unit. To this end, the internal control cell is composed of function block state report information, state monitor information, connection registration information, connection release information, etc.
The signal cell is composed of call control signaling information, connection information, connection accept information, routing information, etc. for point to point connection setup, release information, release completion information, etc. for set-up connection release, and state information, state query information, restart information, restart accept information, etc. for connection management.
The signal cell is further composed of counterpart addition information, counterpart addition accept information, etc. for point to multi-point connection setup, and counterpart removal information, counterpart removal accept information, etc. for set-up connection release.
The control cell and signal cell are important signals for a switching operation of the ATM exchange for transfer of the user cell to a destination. The control cell and signal cell are transferred and processed along the same paths in some parts of the ATM exchange, and along different paths in the other parts thereof.
In other words, the user cell is switched and transferred to an associated destination through a processing operation of an ATM exchange level, not processed by an associated controller in a subscriber interface of the ATM exchange.
The signal cell and control cell, which are composed of the header cell contents as mentioned above, are processed by associated controllers in the subscriber interface of the ATM exchange.
The signal cell, which constitutes the ATM cell together with the user cell, is outputted from a different ATM exchange and then inputted to the subscriber interface of the ATM exchange over a physical layer.
The subscriber interface separates the signal cell from the input ATM cell consisting of the signal cell and user cell, and transfers it to the associated controller for processing thereof.
The control cell, which is defined in the ATM exchange, is transferred to a switch of the ATM exchange together with the user cell. The switch separates the control cell from the user cell and transfers it to a main controller of the ATM exchange.
Now, a description will be given of a conventional method for processing control cells in an ATM exchange with reference to the accompanying drawings.
FIGS. 1 to 3 illustrate the conventional control cell processing method, wherein FIG. 1 is a functional block diagram of a conventional ATM exchange, FIG. 2 is a detailed functional block diagram of a subscriber unit in FIG. 1, and FIG. 3 is a flow chart illustrating the conventional control cell processing method.
With reference to FIG. 1, the conventional ATM exchange comprises a main controller 1 for monitoring respective function blocks of the ATM exchange and generating control signals associated with the respective function blocks as a result of the monitoring, a plurality of subscriber units 3 and 7 each connected to a subscriber or a different ATM exchange for transmitting and receiving ATM cells thereto/therefrom, and a switch 5 for switching an output signal from each of the subscriber units 3 and 7 to an associated path in response to a corresponding control signal from the main controller 1.
With reference to FIG. 2, the subscriber unit, for example, 3 includes a controller 10 for monitoring and managing the entire operation of the subscriber unit and outputting control signals and processed signals, and a first-in first-out memory (FIFO) unit 30 for temporarily storing and outputting input signal cells in an FIFO manner in response to an associated control signal. The FIFO units 30 may include, for example, eight FIFOs.
The subscriber unit 3 further includes a FIFO manager 20 for transmitting and receiving signals to/from the controller 10 according to a Utopia level-2, which is a standard protocol proposed in ITU-T (the successor of CCITT). The FIFO manager 20 generates the above control signal to the FIFO unit 30 in response to a control signal from the controller 10 to control and manage the input and output of signals in the FIFO unit 30. The FIFO manager 20 is also adapted to input or output signals from/to the FIFO unit 30.
With reference to FIG. 3, the conventional subscriber unit cell processing method comprises steps ST11 and ST12 of sequentially checking the eight FIFOs of the FIFO unit 30 to determine whether a new signal cell has arrived at the FIFO unit 30, and steps ST13 and ST14 of, if it is determined at the above step ST12 that the new signal cell has arrived, storing in a register of the FIFO manager 20 a FIFO address in which the contents of the new signal cell are stored and determining whether the transmission of a different signal cell has been completed.
The conventional cell processing method further comprises steps ST15 and ST16 of, if it is determined at the above step ST14 that the transmission of the different signal cell has been completed, selecting a FIFO address of the FIFO unit 30 at which the first signal cell has arrived, setting the selected FIFO address as a physical address and determining whether signal cell contents from the set physical address have been received, and step ST17 of, if it is determined at the above step ST16 that the signal cell contents from the set physical address have been received, monitoring whether the subsequent new signal cell has arrived at the FIFO unit 30.
The above steps ST11–ST17 are performed by the controller 10 in the subscriber unit 3.
A detailed description will hereinafter be given of the conventional subscriber unit cell processing method with reference to FIG. 3.
A cell applied to the ATM exchange subscriber unit 3 is temporarily stored in a connected one of the eight FIFOs of the FIFO unit 30.
The controller 10 in the subscriber unit 3 sequentially checks the eight FIFOs of the FIFO unit 30 (ST11) to determine whether a new signal cell of an ATM cell has arrived at the FIFO unit 30 (ST12).
Upon determining at the above step ST12 that the new signal cell has arrived, the controller 10 stores in the register of the FIFO manager 20 a FIFO address in which the contents of the new signal cell are stored (ST13) and determines whether the transmission of a different cell has been completed (ST14).
Note that the FIFO unit 30 has eight FIFOs to accommodate two subscriber ports, and the FIFO manager 20 and the controller 10 share one Utopia data bus with each other to transmit and receive signals therebetween.
Accordingly, the FIFO manager 20 and the controller 10 have to transmit or receive one signal cell or control cell at one time. Provided that the FIFO manager 20 and the controller 10 desire to transmit or receive a plurality of signal cells or control cells at one time, the controller 10 has to determine and manage priorities of the control cells or signal cells with respect to the FIFO unit 30 through the FIFO manager 20.
In the case where it is determined at the above step ST14 that the transmission of the different cell has been completed, or that no transmission is conducted, the controller 10 selects a FIFO address of the FIFO unit 30 at which the first signal cell has arrived, from the register of the FIFO manager 20, and sets the selected FIFO address as a physical address to receive signal cell contents from the physical address (ST15). The controller 10 then determines whether all the signal cell contents from the physical address have been received (ST16).
If it is determined at the above step ST16 that all the signal cell contents from the set physical address have been received, the controller 10 monitors whether the subsequent new cell has arrived at the FIFO unit 30.
The above-stated control operation of the controller 10 will hereinafter be described in more detail.
1. The controller 10 and the FIFO manager 20 transmit and receive signals therebetween on the basis of a Utopia level-2 that is an ITU-T standard protocol.
2. The controller 10 acts as a master for determining priorities of signals to be transmitted and received.
3. The FIFO manager 20 manages the FIFO unit 30 having a total of eight FIFOs based on two subscriber signals, a signal cell reception_0 FIFO, IPC cell reception_0 FIFO, signal cell transmission_0 FIFO, IPC cell transmission_0 FIFO, signal cell reception_1 FIFO, IPC cell reception_1 FIFO, signal cell transmission_1 FIFO and IPC cell transmission_1 FIFO.
4. While transmitting a signal cell or control cell, the controller 10 sequentially checks the four reception FIFOs of the FIFO unit 30 to determine whether a new signal cell or control cell has arrived.
5. If the new cell has arrived at the FIFO unit 30, then the controller 10 stores a FIFO address of the new cell in the internal register of the FIFO manager 20.
6. If the controller 10 has completed transmission, then it selects a FIFO address of the FIFO unit 30 at which the first cell has arrived and sets the selected FIFO address as a physical address to receive cell contents from the physical address.
7. If the controller 10 starts to receive the cell contents, then it checks the FIFOs beginning with the FIFO subsequent to the current reception FIFO to determine whether a new cell has arrived.
The above-described conventional cell processing method is performed very well when cells are transmitted and received normally without errors.
However, the conventional cell processing method has neither an erroneous cell discard function nor erroneous cell recovery function. For this reason, where a signal cell or control cell is abnormally applied due to hardware problems and other various unusual situations (duplexed switching, noise signal input, misalignment of synchronous clocks and so forth), it cannot be discarded or recovered.
As a result, even though a normal cell has arrived, it is transferred to the controller 10 and in turn the ATM exchange main controller 1 in a continuously abnormal state.
Provided that the above state is maintained for a lengthy period of time, the ATM exchange main controller 1 will manage the subscriber unit 3 or 7 interfaced with the associated subscriber, in an abnormal state or operation disable state, resulting in a significant degradation in stability of the subscriber unit.